1. Field of the Invention
The present invention generally relates to the production of printing plates, and more particularly to a method and system for transferring images by exposing photo-sensitive printing plates.
2. Discussion of the Background
In the graphic arts and printing industries, high-resolution images are formed by exposing a light-sensitizable medium, such as a printing plate, with an appropriate light pattern. By light is meant electromagnetic radiation such as UV, visible-light, or IR radiation. Traditionally, image transfer to printing plates is accomplished by covering the plate with a patterned film and exposing the plate through the film with broadband electromagnetic radiation, e.g., UV or visible light. Thus, for example, the illuminated areas can induce curing of a polymeric light-sensitizable medium. The exposed plates are then processed to remove the unexposed areas, resulting in cured, raised portions that can accept ink for printing.
More recently, systems have been developed that do not use film intermediaries. One such system is the so-called computer-to-plate (CTP) system, where computer generated or processed images are transferred to a printing plate without an intermediary film. Such methods use plates having a light-sensitizable medium, including but not limited to traditional plates, with an integral mask front-surface. Image transfer to the CTP plates is performed by ablating the mask in a pattern corresponding to the image to be printed with electromagnetic radiation, e.g., IR from a digital laser imaging system.
It is a common feature of most CTP imaging systems that the ablation of any specific location on a CTP plate is accomplished by the modulation of the intensity of light beams on the surface. The modulation of the light beam on the surface can occur, for example, by modulating the intensity of a light beam as it scans the surface. In addition, a CTP imaging system may have multiple beams that each ablate different portions, sections, or scan lines of a CTP plate.
In one prior art system, for example, a digital imaging system includes a drum on which the plate is mounted and can rotate across the focal area of an imaging system. With the drum rapidly rotating, the focal area of the light source via the imaging system advances relative to the plate on the drum along the drum's longitudinal direction of the drum. Typically, such longitudinal direction is called the slow scan direction and the direction of motion of the focal area on the plate along the circumference as the drum rotates is called the fast scan direction and is substantially perpendicular to the slow scan direction. The illumination intensity is varied with time, resulting in an ablated mask having the desired image pattern. The plates are then exposed and processed using techniques similar to those of traditional plates.
One measure of the productivity of a CTP system is the rate at which the imaging system can transfer an image to the printing plate. Improved productivity of CTP systems has been achieved by increasing the power of each imaging spot focused on a plate, and also by increasing the number of imaging spots of the imaging system. Thus, for example, early polymer plate CTP systems included a single laser-beam imaging system with a power of 0.5 Watts and were capable of scanning the plate's surface at 0.1 m2/hour. The productivity of prior art CTP imaging systems has typically been improved by increasing the number of laser beams in an imaging system. Thus, for example, a 64 laser-beam imaging system with a power of 30-40 Watts is capable of scanning the plate surface at up to 4-5 m2/hour.
Several approaches have been attempted to improve the productivity of CTP systems. A first approach includes the use of laser diodes each having controllable, modulated powers, where the lasers are arranged either as single emitters or as individual addressable bars. Improved productivity of systems using this approach is provided by increasing the number of emitters. A second approach includes using a plurality of adjacent fiber lasers beams each modulated by their own acousto-optical modulator. A third approach uses one fiber laser beam and an acousto-optical deflector to deflect the beam in a several directions by powering the deflector with several different frequencies at the same time.
While the use of each of these approaches can improve CTP productivity, each approach is limited and has problems that prevent significant improvements over the prior art. For approaches using multiple lasers, it is difficult to provide the required optics at the close spacing required for high resolution CTP systems. The high resolution also requires that each beam is of high quality, adding to the expense of the light source. Another common problem is that of the condensation of ablated material back onto the plate. As the ablated material expands from the plate surface, it can condense on surrounding areas, resulting in residual mask material that affects the quality of the plate.
Thus there is a need in the art for a method and apparatus that permits for an increased scanning speed of CTP digital imaging systems. Such a method and apparatus should be compatible with prior art printing plates, should increase productivity without reducing imaging resolution, and should be inexpensive and easy to implement and control.